The following documents are hereby incorporated by reference: U.S. Provisional Patent Application Serial No. 60/309,608 by Henry A. Hill, filed Aug. 2, 2001 (Z-336), U.S. Provisional Patent Application Serial No. 60/314,345 by Henry A. Hill, filed Aug. 23, 2001 (Z-343), U.S. Provisional Patent Application Serial No. 60/314,568 by Henry A. Hill, filed Aug. 23, 2001 (Z-345), U.S. Provisional Patent Application Serial No. 60/352,341 by Henry A. Hill, filed on Jan. 28, 2002 (Z-391), U.S. Provisional Patent Application Serial No. 60/352,425 by Henry A. Hill, filed on Jan. 28, 2002 (Z-396), and U.S. patent application Ser. No. 10/227,167 by Henry A. Hill, filed on Aug. 23, 2002 (Z345).
This invention relates to interferometers, e.g., displacement measuring and dispersion interferometers that measure angular and linear displacements of a measurement object such as a mask stage or wafer stage in a lithography scanner or stepper system.
Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that subsequently passes through a polarizer.
The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a xe2x80x9cheterodyndxe2x80x9d signal having a beat frequency equal to the difference between the frequencies of the exit measurement and reference beams. If the lengths of the measurement and reference paths are changing relative to one another, e.g., by translating a stage that includes the measurement object, the measured beat frequency includes a Doppler shift equal to 2vnp/xcex, where xcexd is the relative speed of the measurement and reference objects, xcex is the wavelength of the measurement and reference beams, n is the refractive index of the medium through which the light beams travel, e.g., air or vacuum, and p is the number of passes to the reference and measurement objects. Changes in the relative position of the measurement object correspond to changes in the phase of the measured interference signal, with a 2xcfx80 phase change substantially equal to a distance change L of xcex/(np), where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object.
Unfortunately, this equality is not always exact. In addition, the amplitude of the measured interference signal may be variable. A variable amplitude may subsequently reduce the accuracy of measured phase changes. Many interferometers include non-linearities such as what are known as xe2x80x9ccyclic errorsxe2x80x9d. The cyclic errors can be expressed as contributions to the phase and/or the intensity of the measured interference signal and have a sinusoidal dependence on the change in optical path length pnL. In particular, the first harmonic cyclic error in phase has a sinusoidal dependence on (2xcfx80pnL)/xcex and the second harmonic cyclic error in phase has a sinusoidal dependence on 2(2xcfx80pnL)/xcex. Higher harmonic cyclic errors can also be present.
There are also xe2x80x9cnon-cyclic non-linearitiesxe2x80x9d such as those caused by a change in lateral displacement (i.e., xe2x80x9cbeam shearxe2x80x9d) between the reference and measurement beam components of an output beam of an interferometer when the wavefronts of the reference and measurement beam components have wavefront errors. This can be explained as follows.
Inhomogeneities in the interferometer optics may cause wavefront errors in the reference and measurement beams. When the reference and measurement beams propagate collinearly with one another through such inhomogeneities, the resulting wavefront errors are identical and their contributions to the interferometric signal cancel each other out. More typically, however, the reference and measurement beam components of the output beam are laterally displaced from one another, i.e., they have a relative beam shear. Such beam shear causes the wavefront errors to contribute an error to the interferometric signal derived from the output beam.
Moreover, in many interferometry systems beam shear changes as the position or angular orientation of the measurement object changes. For example, a change in relative beam shear can be introduced by a change in the angular orientation of a plane mirror measurement object. Accordingly, a change in the angular orientation of the measurement object produces a corresponding error in the interferometric signal.
The effect of the beam shear and wavefront errors will depend upon procedures used to mix components of the output beam with respect to component polarization states and to detect the mixed output beam to generate an electrical interference signal. The mixed output beam may for example be detected by a detector without any focusing of the mixed beam onto the detector, by detecting the mixed output beam as a beam focused onto a detector, or by launching the mixed output beam into a single mode or multi-mode optical fiber and detecting a portion of the mixed output beam that is transmitted by the optical fiber. The effect of the beam shear and wavefront errors will also depend on properties of a beam stop should a beam stop be used in the procedure to detect the mixed output beam. Generally, the errors in the interferometric signal are compounded when an optical fiber is used to transmit the mixed output beam to the detector.
Amplitude variability of the measured interference signal can be the net result of a number of mechanisms. One mechanism is a relative beam shear of the reference and measurement components of the output beam that is for example a consequence of a change in orientation of the measurement object.
In dispersion measuring applications, optical path length measurements are made at multiple wavelengths, e.g., 532 nm and 1064 nm, and are used to measure dispersion of a gas in the measurement path of the distance measuring interferometer. The dispersion measurement can be used in converting the optical path length measured by a distance measuring interferometer into a physical length. Such a conversion can be important since changes in the measured optical path length can be caused by gas turbulence and/or by a change in the average density of the gas in the measurement arm even though the physical distance to the measurement object is unchanged.
The interferometers described above are often crucial components of scanner systems and stepper systems used in lithography to produce integrated circuits on semiconductor wafers. Such lithography systems typically include a translatable stage to support and fix the wafer, focusing optics used to direct a radiation beam onto the wafer, a scanner or stepper system for translating the stage relative to the exposure beam, and one or more interferometers. Each interferometer directs a measurement beam to, and receives a reflected measurement beam from, a plane mirror attached to the stage. Each interferometer interferes its reflected measurement beams with a corresponding reference beam, and collectively the interferometers accurately measure changes in the position of the stage relative to the radiation beam. The interferometers enable the lithography system to precisely control which regions of the wafer are exposed to the radiation beam.
In many lithography systems and other applications, the measurement object includes one or more plane mirrors to reflect the measurement beam from each interferometer. Small changes in the angular orientation of the measurement object, e.g., pitch and yaw of a stage, can alter the direction of each measurement beam reflected from the plane mirrors. If left uncompensated, the altered measurement beams reduce the overlap of the exit measurement and reference beams in each corresponding interferometer. Furthermore, these exit measurement and reference beams will not be propagating parallel to one another nor will their wave fronts be aligned when forming the mixed beam. As a result, the interference between the exit measurement and reference beams will vary across the transverse profile of the mixed beam, thereby corrupting the interference information encoded in the optical intensity measured by the detector.
To address this problem, many conventional interferometers include a retroreflector that redirects the measurement beam back to the plane mirror so that the measurement beam xe2x80x9cdouble passesxe2x80x9d the path between the interferometer and the measurement object. The presence of the retroreflector insures that the direction of the exit measurement is insensitive to changes in the angular orientation of the measurement object. When implemented in a plane mirror interferometer, the configuration results in what is commonly referred to as a high-stability plane mirror interferometer (HSPMI). However, even with the retroreflector, the lateral position of the exit measurement beam remains sensitive to changes in the angular orientation of the measurement object. Furthermore, the path of the measurement beam through optics within the interferometer also remains sensitive to changes in the angular orientation of the measurement object.
In practice, the interferometry systems are used to measure the position of the wafer stage along multiple measurement axes. For example, defining a Cartesian coordinate system in which the wafer stage lies in the x-y plane, measurements are typically made of the x and y positions of the stage as well as the angular orientation of the stage with respect to the z axis, as the wafer stage is translated along the x-y plane. Furthermore, it may be desirable to also monitor tilts of the wafer stage out of the x-y plane. For example, accurate characterization of such tilts may be necessary to calculate Abbe offset errors in the x and y positions. Thus, depending on the desired application, there may be up to five degrees of freedom to be measured. Moreover, in some applications, it is desirable to also monitor the position of the stage with respect to the z-axis, resulting in a sixth degree of freedom.
To measure each degree of freedom, an interferometer is used to monitor distance changes along a corresponding metrology axis. For example, in systems that measure the x and y positions of the stage as well as the angular orientation of the stage with respect to the x, y, and z axes, at least three spatially separated measurement beams reflect from one side of the wafer stage and at least two spatially separated measurement beams reflect from another side of the wafer stage. See, e.g., U.S. Pat. No. 5,801,832 entitled xe2x80x9cMethod of and Device for Repetitively Imaging a Mask Pattern on a Substrate Using Five Measuring Axes,xe2x80x9d the contents of which are incorporated herein by reference. Each measurement beam is recombined with a reference beam to monitor optical path length changes along the corresponding metrology axes. Because the different measurement beams contact the wafer stage at different locations, the angular orientation of the wafer stage can then be derived from appropriate combinations of the optical path length measurements. Accordingly, for each degree of freedom to be monitored, the system includes at least one measurement beam that contacts the wafer stage. Furthermore, as described above, each measurement beam may double-pass the wafer stage to prevent changes in the angular orientation of the wafer stage from corrupting the interferometric signal. The measurement beams may generated from physically separate interferometers or from multi-axes interferometers that generate multiple measurement beams.
The invention features a multiple-degrees of freedom measuring plane mirror interferometer assembly measures two, three or more degrees of freedom with either zero or substantially reduced differential beam shear at one or more detectors or fiber optic pickups (FOP). In certain embodiments of the present invention, the differential beam shear of reference and measurement beams in one or more interferometers of the interferometer assembly are substantially reduced. The interferometer assembly may include a single interferometer optical assembly. A two-degrees of freedom measuring plane mirror interferometer assembly with zero or substantially reduced differential beam shear at one detector or FOP may be configured to measure two linear displacements of two separated locations on a plane mirror or measure both a linear displacement and an angular displacement of a plane mirror. In certain of the configurations, the differential beam shear of reference and measurement beams in one of the corresponding interferometers of the configuration is substantially reduced.
The techniques described herein may be extended to measure additional degrees of freedom using the interferometer configurations disclosed in U.S. patent application Ser. No. 60/352,341 by Henry A. Hill and filed on Jan. 28, 2002 (Z-391), which was incorporated by reference. Such embodiments include a three-degrees of freedom measuring plane mirror interferometer assembly with zero or substantially reduced differential beam shear at one or more detectors and/or FOP""s may be configured to measure three linear displacements of three separated locations on a plane mirror or a linear displacement and two orthogonal angular displacements of a plane mirror or to measure two linear displacements and one angular displacement. In certain of the configurations, the differential beam shear of reference and measurement beams in one or more of the corresponding interferometers of the configuration are substantially reduced. Further embodiments include a four or more degrees of freedom measuring plane mirror interferometer assembly with zero or reduced differential beam shear at one or more detectors and/or FOP""s may be configured to measure other combinations of linear and angular displacements. In certain of the configurations, the differential beam shear of reference and measurement beams in one or more of the corresponding interferometers of the certain of the configurations are substantially reduced.
A single plane mirror may be used both as the reference object and measurement object in the measurement of changes in orientation of an object. The reference and measurement beams used in the measurement of angles may make single passes to the single plane mirror. The interferometer optical assembly may include high stability configurations for either or both the linear and angular displacement interferometers. The interferometer optical assembly may be configured so that there is either zero or reduced differential beam shear between the reference and measurement beam components at a detector or FOP for beams used in measurement of an angle. The beam shears of the reference and measurement beams at the single plane mirror are zero for the reference and measurement beams used in the angular displacement interferometers. Two or more linear and angular displacement output beams may have a common measurement beam path in a pass to the single plane mirror. The interferometer assemblies may be configured so that the respective reference and measurement beam optical path lengths of the linear and angular displacement interferometers are of equal lengths in glass and/or equal lengths in a gas.
In general, in one aspect, the invention features an apparatus that includes a multiple-pass interferometer. The multiple-pass interferometer includes reflectors to reflect at least two beams along multiple passes through the interferometer, the multiple passes including a first set of passes and a second set of passes. The reflectors have first alignments that are normal to the directions of the paths of the beams that are reflected by the reflectors. The two beams provide information about changes in a first location on one of the reflectors after the first set of passes. The two beams provide information about changes in the first location and changes in a second location on the reflector after the second set of passes. The paths of the beams are sheared during the first set of passes and during the second set of passes if at least one of the reflectors has an alignment other than the first alignment. The interferometer includes optics to redirect the beams after the first set of passes and before the second set of passes so that shear imparted during the second set of passes cancels shear imparted during the first set of passes.
Embodiments of the apparatus may include one or more of the following features.
The optics are configured to redirect the beams while maintaining the magnitude and direction of shear between the two beams. The propagation path of one of the two beams after being redirected by the optics is parallel to the propagation path of the other one of the two beams after completing the first set of passes. The reflectors include plane reflection surfaces. The beams include a reference beam that is directed toward one of the reflectors maintained at a position that is stationary relative to the interferometer. The beams include a measurement beam that is directed towards one of the reflectors that is movable relative to the interferometer. The paths of the reference and measurement beams define an optical path length difference, the changes in the optical path length difference indicative of changes in the position of the one of the reflectors that is movable relative to the interferometer. The reflectors include a first reflector and a second reflector, the beams comprising a first beam directed toward the first reflector and a second beam directed toward the second reflector, each of the first and second reflectors being movable relative to the interferometer.
The paths of the first and second beams define an optical path length difference, the changes in the optical path length difference indicative of changes in relative positions of the first and second reflectors. The first set of passes consists of two passes, and during each pass each of the beams is reflected by one of the reflectors at least once. The second set of passes consists of two passes, and during each pass each of the beams is reflected by one of the reflectors at least once. The multiple-pass interferometer includes a beam splitter that separates an input beam into the beams and directs the beams toward the reflectors. The beam splitter includes a polarizing beam splitter. The optics may include an odd number of reflection surfaces. Normals of the reflection surfaces lie in a common plane. The reflection surfaces include plane reflection surfaces.
For each beam redirected by the optics, the beam is reflected by the reflection surfaces such that a sum of angles between incident and reflection beams of each reflection surface is zero or an integer multiple of 360 degrees, the angle measured in a direction from the incident beam to the reflection beam, the angle having a positive value when measured in a counter clockwise direction and a negative value when measured in a clockwise direction.
The interferometer combines the beams after the beams travel through the first and second set of passes to form overlapping beams that exit the interferometer. The optics may consist of one reflection surface. The optics may include an even number of reflection surfaces. The optics include a cube corner retroreflector. The interferometer includes a differential plane mirror interferometer. The two beams have different frequencies.
In general, in another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes a detector that responds to optical interference between the overlapping beams and produces an interference signal indicative of an optical path length difference between the paths of the beams. The detector include a photodetector, an amplifier, and an analog-to-digital converter. An analyzer is coupled to the detector to estimate a change in an optical path length difference of the beams based on the interference signal. A source provides the beams.
In general, in another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes a stage to support the wafer, an illumination system to image spatially patterned radiation onto the wafer, a positioning system to adjust the position of the stage relative to the imaged radiation, and any of the interferometetric apparatuses described above. The interferometric apparatus include an interferometer for measuring the position of the stage relative to the patterned radiation.
In general, in another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes a stage to support a wafer for fabricating integrated circuits thereon, and an illumination system including a radiation source, a mask, a positioning system, a lens assembly, and any of the interferometric apparatuses described above. During operation, the source directs radiation through the mask to produce spatially patterned radiation, the positioning system adjusts the position of the mask relative to the radiation from the source, the lens assembly images the spatially patterned radiation onto the wafer, and the interferometer of the interferometric apparatus is used to monitor the position of the mask relative to the wafer.
In general, in another aspect, the invention features a lithography system for use in fabricating a lithography mask. The lithography system includes a source to provide a write beam to pattern a lithography mask, a stage to support the lithography mask, a beam directing assembly to deliver the write beam to the lithography mask, a positioning system to position the stage and beam directing assembly relative to one another, and any of the interferometric apparatuses described above. The interferometric apparatus includes an interferometer for measuring the position of the stage relative to the beam directing assembly.
Integrated circuits may be fabricated by using any of the lithography systems described above to support a wafer, image spatially patterned radiation onto the wafer, and adjust the position of the stage relative to the imaged radiation, in which the interferometer is used to measure the position of the stage.
Integrated circuits may be fabricated by using any of the lithography systems described above to support a wafer, direct radiation from the source through a mask to produce spatially patterned radiation on the wafer, adjust the position of the mask relative to the radiation from the source, and image the spatially patterned radiation onto the wafer. The interferometer of the lithography system is used to measure the position of the mask relative to the wafer.
Lithography masks may be fabricated by using any of the lithography systems described above to support the lithography mask, deliver a write beam to the lithography mask, and position the stage and beam directing assembly relative to one another. The interferometer of the lithography system is used to measure the position of the stage relative to the beam directing assembly.
In general, in another aspect, the invention features a method, including directing a first measurement beam along a first set of passes through an interferometer to a first region on a measurement object, directing a first reference beam along a first set of passes through the interferometer to a reference object, combining the first measurement and reference beams to produce a first output beam after the first measurement and reference beams complete the first set of passes, determining a change in position in the first region on the measurement object, using optics to direct a portion of the first measurement beam to form a second measurement beam, using the optics to direct a portion of the first reference beam to form a second reference beam, directing the second measurement beam along a second set of passes through the interferometer to a second region on the measurement object, directing the second reference beam along a second set of passes through the interferometer to the reference object, combining the second measurement and reference beams to produce a second output beam after the second measurement and reference beams complete the second set of passes, and determining a change in position in the second region on the measurement object. A rotation of the measurement object relative to the directions of the paths of the first and second measurement beams that are incident on the measurement object imparts beam shear to the first measurement beam during the first set of passes and to the second measurement beam during the second set of passes. The optics are configured to redirect the portion of the first measurement and reference output beams so that shear imparted upon the second measurement beam during the second set of passes cancels shear imparted upon the first measurement beam during the first set of passes.
The method may further include additional features corresponding to any of the features described above in connection with the different apparatuses.
In general, in another aspect, the invention features a lithography method for use in fabricating integrated circuits on a wafer. The method includes supporting the wafer on a moveable stage, imaging spatially patterned radiation onto the wafer, adjusting the position of the stage relative to the imaged radiation, and monitoring the position of the stage relative to the imaged radiation using any of the interferometric methods described above.
In general, in another aspect, the invention features a lithography method for use in fabricating integrated circuits on a wafer. The method includes directing input radiation through a mask to produce spatially patterned radiation, positioning the mask relative to the input radiation, monitoring the position of the mask relative to the input radiation using any of the interferometric methods described above, and imaging the spatially patterned radiation onto the wafer.
In general, in another aspect, the invention features a lithography method for use in fabricating integrated circuits on a wafer. The method includes positioning a first component of a lithography system relative to a second component of a lithography system to expose the wafer to spatially patterned radiation, and monitoring the position of the first component relative to the second component using any of the interferometric methods described above.
In general, in another aspect, the invention features a lithography method for use in fabricating a lithography mask. The method includes directing a write beam to a substrate to pattern the substrate, positioning the substrate relative to the write beam, and monitoring the position of the substrate relative to the write beam using any of the interferometric methods described above.
In general, in another aspect, the invention features an apparatus including a multiple degree of freedom interferometer for measuring changes in position of a measurement object with respect to multiple degrees of freedom. The interferometer is configured to receive an input beam, direct a first-measurement beam derived from the input beam to make first and second passes to the measurement object about a first point on the measurement object, and then combine the first measurement beam with a first reference beam derived from the input beam to produce a first output beam comprising information about changes in distance to the first point on the measurement object. The interferometer is further configured to direct a second-measurement beam derived from the input beam to make first and second passes to the measurement object about a second point on the measurement object, and then combine the second measurement beam with a second reference beam derived from the input beam to produce a second output beam comprising information about changes in distance to the second point on the measurement object. The interferometer includes fold optics positioned to reflect a portion of the first output beam an odd number of times in a plane defined by the incidence of the measurement beams on the measurement objects to define a secondary input beam. The second measurement beam and the second reference beam are derived from the secondary input beam.
Embodiments of the apparatus may include one or more of the following features.
The interferometer includes a polarizing beam splitter for directing the different beams along their respective paths, a first quarter wave plate positioned between the polarizing beam splitter and a reference object, and a second quarter wave plate positioned between the polarizing beam splitter and the measurement object. The reference object includes a plane mirror oriented substantially normal to the incident beam portions. The fold optics include a non-polarizing beam splitter positioned to separate a portion of the first output beam to define the secondary input beam and direct it back to the polarizing beam splitter to produce the second measurement beam and the second reference beam. The fold optics include a plurality of reflective surfaces positioned to direct the secondary input beam from the non-polarizing beam splitter to the polarizing beam splitter, and wherein the non-polarizing beam splitter and the plurality of reflective surfaces reflect the second input beam an odd number of times prior to reaching the polarizing beam splitter. The non-polarizing beam splitter reflects the first output beam to produce the secondary input beam, and the plurality of reflective surfaces reflects the secondary input beam an even number of times. The apparatus includes a first fiber optic pickup for coupling the first output beam to a detector and a second fiber optic pickup for coupling the second output beam to a detector.
In general, in another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes a stage for supporting the wafer, an illumination system for imaging spatially patterned radiation onto the wafer, a positioning system for adjusting the position of the stage relative to the imaged radiation, and any of the interferometric apparatuses described above for monitoring the position of the wafer relative to the imaged radiation.
In general, in another aspect, the invention features a lithography system for use in fabricating integrated circuits on a wafer. The lithography system includes a stage for supporting the wafer, and an illumination system including a radiation source, a mask, a positioning system, a lens assembly, and any of the interferometric apparatuses described above. During operation, the source directs radiation through the mask to produce spatially patterned radiation, the positioning system adjusts the position of the mask relative to the radiation from the source, the lens assembly images the spatially patterned radiation onto the wafer, and the interferometer of the interferometric apparatus monitors the position of the mask relative to the radiation from the source.
In general, in another aspect, the invention features a beam writing system for use in fabricating a lithography mask. The system includes a source providing a write beam to pattern a substrate, a stage supporting the substrate, a beam directing assembly for delivering the write beam to the substrate, a positioning system for positioning the stage and beam directing assembly relative one another, and any of the interferometric apparatuses described above for monitoring the position of the stage relative to the beam directing assembly.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict with publications, patent applications, patents, and other references mentioned incorporated herein by reference, the present specification, including definitions, will control.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.